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Controlled permeability formwork
Concrete cast against sealed timber or steel formwork has a reduced cement content and increased water/cement ratio (concrete is less dense and more porous) compared to concrete located beyond the cover zone. During compaction of the concrete, excess water and air are driven towards the formwork where they become trapped, resulting in blowholes and other surface blemishes.
Controlled Permeability Formwork (CPF) consists of a filter and drainage layer attached to the formwork, which allows the water in contact with the formwork to drain away. The result is a blow-hole free surface that has the additional benefit of having a lower w/c ratio and therefore higher resistance to aggressive substances, increasing the life of the structure.
Acknowledgement:
The Concrete Society
Concrete Bookshop - Members receive 40% discount on Concrete Society publications
Formwork - a guide to good practice, 3rd edition
Visual Concrete: Control of blemishes
